Structural, energetic, and electronic properties of acetonitrile (ACN) clusters (CH3CN)n, where n=2–8 is the number of ACN molecules, have been investigated by density functional theory. The structure of ACN clusters can be associated with the replica of building units, which may involve dimers, trimers, or tetramers. Two types of building units can be identified: structures that are mainly stabilized by CH⋯N interactions; structures where dipolar interactions are dominant. We are providing evidence that competition between weak hydrogen bonding and dipolar interactions may determine the structure of ACN clusters. In comparison with typical hydrogen bonding systems (e.g. (H2O)n, (HF)n), nonadditive polarization effects are much less important. The average monomer dipole moment μ̄ in ACN clusters tends to values in the 4.5–4.7D range for larger aggregates. This result for μ̄ is in very good agreement with an experimental prediction for the dipole moment of liquid acetonitrile of 4.5±0.1D [Mol. Phys. 73 (1991) 985].